The invention relates to laser power supplies and, more specifically to low-noise power supplies for laser diodes.
In semiconductor lasers, particularly CW-operated laser diodes (Continuous Wave, or continuous mode), power supply induced noise currents manifest themselves as corresponding instabilities in output level and wavelength. Accordingly, CW laser diodes typically require an accurate, low-noise current source to achieve high stability. Due to the high power levels often required of laser power supplies, it is common practice to use switch-mode power supplies to maximize efficiency. However, it is well-known that such switching power supplies generate considerable noise and high output ripple as compared to xe2x80x9cquieterxe2x80x9d but less efficient linear supplies.
To overcome this problem, a linear pass element connected as a current driver is usually employed in series with xe2x80x9crawxe2x80x9d power supply output and the laser diode load. An example of such an arrangement 100 is shown in FIG. 1. A voltage regulated xe2x80x9crawxe2x80x9d or xe2x80x9cbulkxe2x80x9d power supply 102 provides power for a load comprising one or more laser diodes 104 (e.g., an array or diodes). Typically, the power supply 102 is a switching power supply. The output of the power supply 102 is smoothed by a capacitor 106. A ground-referenced current source 108 comprising a linear pass element 110, a current sensing element 112 and an error amplifier 114 controls the amount of current conducted through the diode load. The linear pass element 110, typically a FET (field-effect transistor), conducts current from the power supply 102 through the laser diode(s) 104 into the grounded current sensing element 112. A voltage develops across the current sensing element 112 in proportion to the amount of current being conducted through the laser diodes 104. The error amplifier 114 compares the sensed current to a control voltage that indicates the desired laser diode current and adjusts the current conducted by the linear pass element 110 accordingly to maintain constant current at the desired level. The filtering effect of the capacitor 106, in combination with the ripple and noise rejection of the linear current source 108, improves overall stability and minimizes power supply induced noise.
In operation, with the current source 108 conducting current through the laser diode(s) 104, energy is drawn from the capacitor 106 through the diodes, as a result of which the voltage on the capacitor falls. Therefore, the current source has to have sufficient compliance to continue to maintain current regulation as the xe2x80x9crawxe2x80x9d supply voltage falls. For good efficiency, a low voltage loss across the current source is desired, but this requires a large and bulky capacitor to minimize voltage xe2x80x9cdroopxe2x80x9d.
The disadvantages of such an implementation include:
a) The power dissipated in the linear pass element 110 may be considerable, resulting in substantial heat generation and consequent inefficiency. Heat sinking and cooling may be required, resulting in a large, expensive, inefficient system.
b) All of the laser diode current flows through the linear pass element 110, requiring a high-current device with commensurate size and cost penalties.
c) Laser diodes are presently very expensive. If the series pass element 110 were to fail to a short-circuit condition, then the voltage stored on the capacitor 106 would be applied directly across the laser diode(s) 104, resulting in unregulated current flow, potentially producing excessive light output and possible diode damage.
Another example of a series-connected linear pass element being used to regulate current conducted through laser diode load is disclosed in U.S. Pat. No. 5,287,372 (xe2x80x9cORTIZxe2x80x9d), incorporated in its entirety by reference herein. ORTIZ discloses a zero-current, switched, full wave quasi-resonant converter that provides a current to directly drive the laser diode. Referring to FIG. 2 of ORTIZ, a linear pass element 24 (Q1) is connected in series with the laser diode load 31 and is used to regulate the current conducted therethrough. The laser diode driver circuit described in ORTIZ suffers from the disadvantages described hereinabove with respect to the current driver circuit arrangement of FIG. 1.
It therefore is a general object of the present invention to provide an improved technique for driving laser diodes.
It is a further object of the present invention to create a smaller, less expensive, low-noise current driver for laser diodes without the efficiency loss of a series-connected linear pass element.
It is a further object of the present invention to create a low-noise current driver for laser diodes that can employ less expensive, lower-current devices while maintaining good load regulation.
According to the invention, a low-noise current source driver for a laser diode load comprises a current-regulated supply connected across the load, and a shunt regulator. The shunt regulator comprises a shunting element, a current sensing element for sensing current conducted through the load, and an error amplifier responsive to a difference between the current sensed by the current sensing element and a signal representative of a first reference current. The current regulator is designed to respond to a signal representative of a second reference current to produce an appropriate corresponding output current. The shunting element is connected across the power supply and load, and is controlled by the error amplifier to conduct all current from the current regulated supply in excess of the first reference current. The second reference current is greater than the first reference current. The shunting element may be a field-effect transistor (FET) or a bipolar transistor. The current sensing element may be a small-value resistor or a Hall-effect device.
Generally speaking, the second reference current is always greater than the first reference current by an amount sufficient to ensure that ripple and noise currents cannot cause the current-regulated supply output to dip below the first reference current. This is accomplished in one of three ways:
the second reference current is made greater than the first reference current by a fixed amount;
the second reference current is made greater than the first reference current by a fixed proportion (e.g., percentage); or
the second reference current is made greater than the first reference current by an amount equal to the sum of a fixed proportion of the first reference current and a fixed amount.
Other objects, features and advantages of the invention will become apparent in light of the following description thereof.
Reference will be made in detail to preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. The drawings are intended to be illustrative, not limiting. Although the invention will be described in the context of these preferred embodiments, it should be understood that it is not intended to limit the spirit and scope of the invention to these particular embodiments.
Often, similar elements throughout the drawings may be referred to by similar references numerals. For example, the element 199 in a figure (or embodiment) may be similar or analogous in many respects to an element 199A in another figure (or embodiment). Such a relationship, if any, between similar elements in different figures or embodiments will become apparent throughout the specification, including, if applicable, in the claims and abstract. In some cases, similar elements may be referred to with similar numbers in a single drawing. For example, a plurality of elements 199 may be referred to as 199A, 199B, 199B, etc.